Many of the existing information reproducing apparatuses reproduce information from magnetic disks or optical disks as information recording mediums. In particular, CDs, one of the optical disks, are broadly utilized as media recording a large capacity of information due to the capability of recording information with density and mass production at low cost. The CD has, on its surface, pits formed having a size of nearly a wavelength of the laser light to be used in reproducing and a depth of approximately a quarter of the wavelength, enabling reading out utilizing interference phenomenon of light.
The lens optical systems generally used in the optical microscopes are utilized for reading recorded information out of the optical disks representative of the CDs. Accordingly, where the pit size or track pitch is reduced to increase information recording density, the laser-light spot cannot be reduced in size to a half wavelength or smaller due to the problem with optical diffraction limit, thus running against a wall that the information recording unit is impossible to reduce to a size smaller than the laser-light wavelength.
Meanwhile, without limitation to the optical disks, the magneto-optical disks and DVDs recording information by the magneto-optical recording scheme and phase-shift recording scheme realize recording/reproducing of information with density due to laser-light microscopic spot. Consequently, the information recording density is limited to a spot size obtained by focusing laser light.
In such circumstances, in order to break through such limitation due to diffraction limit, there is a proposal on an information reproducing apparatus using an optical head having a microscopic aperture having a diameter smaller than a wavelength of the laser light utilized in reproducing, e.g. nearly one-tenth thereof, to utilize near-field light (including both near field and far field) produced in the microscopic aperture.
Conventionally, as an apparatus utilizing near-field light, there has been a near-field microscope using a probe having the above microscopic aperture which has been utilized for observing a microscopic surface texture of a sample. As one of the near-field light utilizing schemes in the near-field microscope, there is a scheme (illumination mode) that the probe microscopic aperture and a sample surface are approached in distance to nearly a diameter of the probe microscopic aperture to introduce propagation light through the probe and toward the probe microscopic aperture thereby producing near-field light in the microscopic aperture. In this case, the scattering light caused through interaction between the produced near-field light and the sample surface is detected, involving an intensity and phase reflecting sample-surface fine texture, by a scattering-light detecting system. Thus, observation is made possible with a high resolving power never realized in the convention optical microscopes.
Meanwhile, as another scheme of the near-field microscope utilizing near-field light, there is a scheme (collection mode) that propagation light is illuminated to a sample to localize near-field light on a sample surface so that the probe microscopic aperture is approached to the sample surface to a distance of nearly a diameter of the probe microscopic aperture. In this case, the scattering light caused through interaction between the localized near-field light and the probe microscopic aperture is guided, involving an intensity and phase reflecting sample-surface fine texture, to a scattering-light detecting system through probe microscopic aperture, achieving observation with high resolving power.
The foregoing information reproducing apparatus utilizing near-field light makes use of these observation schemes for the near-field microscopes. By utilizing such near-field light, it is possible to reproduce information from the information recording medium having record with density.
In order to realize reproducing of information recorded with density on an information recording medium by utilizing near-field light mentioned above, a positioning control technology is required to accurately move the probe microscopic aperture as an optical head to an arbitrary position on the information recording medium.
In the magnetic disk : apparatuses, a servo-surface servo scheme and a sector servo scheme are generally adopted for position control. The servo-surface servo scheme is a method that one of a plurality of disk surfaces is used exclusively for servo so that a servo magnetic head is positioned relative to this servo surface and the remaining disk surfaces and magnetic heads are used for data. Also, the sector servo scheme is a method that servo information is embedded here and there on the data surface to use discretely-detected servo information thereby positioning the magnetic head to a data track.
However, it is difficult to apply the positioning control employed for the magnetic disk apparatus to positioning control for reproducing with a high-density information medium due to near-field light. For example, because in the servo-surface servo scheme the servo head and the data head are determined in position accuracy by mechanical accuracy, there is a possibility of causing positional deviation between the both heads due to a difference in temperature distribution, thus making it improper to employ as positioning control to an information recording medium with especial density in the information reproducing apparatus utilizing near-field light.
Meanwhile, the above sector servo scheme is free from occurrence of positional deviation of the head by the difference in temperature distribution as is rendered problematic in the servo-surface servo scheme. However, in a control-system design stage, there is a need to handle as a discrete-value system different from the conventional continuous system. Because the information reproducing apparatus utilizing near-field light requires accurate positioning to an especially dense information recording medium, devising is needed for using such a complicated control system.
On the other hand, the optical disk apparatus employs a positioning control method, particularly as tracking error detecting method, a three-beam method, a push-pull method, a pre-wobbling tracking error detecting method. The three-beam method is a method that the beam from a laser diode is divided by a diffraction grating into totally three beams of 0-order light (main beam) for recording and reproducing, ±1-order light (sub-beams) of two in number and two sub-beams are slightly deviated from a center of a guide groove provided on an optical disk so that reflection light from the both is received by two light-receiving surfaces of a photo-detector to provide a differential signal thereof for controlling the objective lens.
Also, the push-pull method is a method that the reflection light of a beam illuminated to a guide groove provided on an optical disk is detected in a two-division detector to thereby obtain a differential signal as a tracking error signal for controlling the objective lens. The pre-wobbling tracking error detecting method is a method that elongate pits (wobbling marks) A and B, each set thereof comprising two, are arranged relative to a center of the track radially of the disk with slight deviation relative to a center of the track so that, when the light spot traces a center of the track, a change in the amount of the reflection light from the pits A and B is caused as a tracking error signal to control the objective lens.
The above-explained tracking error detection methods for the optical disk apparatus in any thereof are the methods wherein the illumination light to the pits formed on the disk and the reflection light reflected by them are both handled as propagation light (far field). Devising has been required where applied for detecting non-propagation light (near field) such as near-field light or reflection scattering light thereof. In particular, in the information recording medium that reproduction is possible with utilization of near-field light, information recording unit could have been determined by a difference in optical property besides concave-convex information such as pits formed on the conventional optical disk. There has been a desire for optical-head positioning control for reproducing from such an information recording medium, particularly an information reproducing apparatus for performing tracking.
It is an object of the present invention to provide, in view of the above problems, an information recording medium and information reproducing apparatus for reproducing information with reliability for an information recording medium having dense record, particularly realizing tracking by a simple structure.